This invention relates to electronic circuits for ballasting discharge lamps. More particularly, this invention relates to high frequency ballast circuits incorporating series resonant starting circuits.
Electric discharge lamps are characterized by a negative resistance volt-ampere characteristic and a relatively high ionization potential which tends to decrease as the lamp electrodes heat during operation. Auxiliary circuits for operating such lamps must provide high open circuit voltage for starting and a positive series impedance for ballasting stable lamp operation. These characteristics are typically obtained by use of a series ballast having high leakage inductance. Such prior art circuits, however, draw substantial amounts of reactive current and are, therefore, generally unsuitable for operating discharge lamps in conjunction with transistorized power inverters.
Lamp ballast circuits of the prior art have incorporated capacitors connected in parallel with the discharge lamp to resonate an inductive ballast and provide high open circuit starting voltage. Such capacitors have been chosen to resonate at the operating frequency of the primary power source and, thus, to achieve increased voltage for lamp ionization. This technique loses effectiveness, however, as the inductive and capacitive components age and the resonant frequency of the starting circuit shifts from that of the power source.